Desalination Using Low-Grade Thermal Energy

ABSTRACT

This invention describes a low temperature, self-sustainable desalination process operated under natural vacuum conditions created and maintained by barometric pressure head.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalApplication Ser. No. 60/950,076, entitled “Desalination using Low-GradeThermal Energy”, to Nagamany Nirmalakhandan et al., filed on Jul. 16,2007, and the specification thereof is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The present invention relates to an apparatus and method fordesalination.

2. Description of Related Art

Interest in the use of low grade heat sources and recovery of waste heatis growing due to increasing energy costs and declining energy sources.Examples of low grade energy sources include solar energy and heatrejected by fossil fuel-based power plants, airconditioning/refrigeration systems, and industrial processes. As aconsequence of the laws of thermodynamics, thermal systems have toreject large quantities of low grade heat energy to the environment. Forexample, heat rejection rate of modern combined cycle power plants isalmost equal to their output. Approaches to utilize waste heat toproduce value added products or services can conserve limited energysources, reduce adverse environmental impacts, and minimize overallcosts.

The present invention utilizes low grade heat to operate a newdesalination process. Traditional desalination processes such as reverseosmosis, electrodialysis, mechanical vapor compression, and multi-effectflash distillation require electrical energy derived from nonrenewablesources, the cost of which has increased by 10 times over the past 20years. Recently, a new desalination process has been proposed that hasthe potential to run solely on low grade heat sources at around 50° C.The present invention is a modification to that process, whereby it canrun around the clock, using a thermal energy storage (TES) system thatenables waste heat sources and renewable energy sources to be used todrive the process with minimum reliance on grid power. TES managesvariable energy demand over time, is a continuous heat source, and has alower specific energy for desalination. The present invention, unlikethe process mentioned above, enables solar collectors and photovoltaicpanels to provide the energy to drive the process. The TES system can bemaintained at the desired temperature using low grade waste heat fromany available source.

BRIEF SUMMARY OF THE INVENTION

A preferred embodiment of the present invention is described below. Theembodiment is a desalination system comprising a desalination unit, alow grade heat source for powering the desalination unit and a thermalenergy storage system for storing the low grade heat source andmaintaining a specified temperature range. The system is able to runcontinuously within a specified temperature range between approximately40-50° C. The low-grade heat source is at least partially supplied by anabsorption refrigeration unit which maintains the thermal energy storagerefrigeration unit at a specified temperature range. The absorptionrefrigeration unit operates at a pressure range of between approximately1.4 to 15.75 kPa.

The desalination unit comprises an evaporation chamber, a condenser, aheat exchanger and one or more columns. The columns comprise a salinewater column, a brine withdrawal column and a desalinated water column.The heat input to the evaporation chamber is provided by the thermalenergy storage system.

The desalination system does not have a pump except for an initialstarting pump. In addition, the system does not have any other movingparts.

Another embodiment of the present invention is a method of desalinatingcomprising the steps of operating a desalination unit, powering thedesalination unit using a low grade heat source, storing the low gradeheat source in a thermal energy storage unit and maintaining a specifiedtemperature range of the low grade heat source. The desalination unitcan run continuously and comprises desalinating saline water at atemperature range of approximately 40-50° C. The method can alsocomprise supplying the low grade heat source at least partially by anabsorption refrigeration unit. The absorption refrigeration unit canalso provide a cooling load. The thermal energy storage unit ismaintained at a specified temperature range within the absorptionrefrigeration unit. Finally, the absorption refrigeration unit operatesat a pressure range of between approximately 1.4 to 15.75 kPa.

Further scope of applicability of the present invention will be setforth in part in the detailed description to follow, taken inconjunction with the accompanying drawings, and in part will becomeapparent to those skilled in the art upon examination of the following,or may be learned by practice of the invention. The objects andadvantages of the invention may be realized and attained by means of theinstrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate one or more embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating one or more preferred embodiments of the invention and arenot to be construed as limiting the invention. In the drawings:

FIG. 1 is a schematic of a preferred embodiment of the desalinationsystem of the present invention;

FIG. 2 is a table of model parameters used in an example simulation ofmass and energy balances of a preferred embodiment;

FIG. 3 is a chart showing heat transfers and efficiency for thedesalination system of the present invention over 24 hours;

FIG. 4 is a chart showing variations of desalinated water temperatureand saline water temperature with respect to ambient temperature over 24hours for the desalination system of the present invention;

FIG. 5 is a comparison of parameters in a typical absorptionrefrigeration system and in the preferred embodiment;

FIG. 6 is a chart showing ambient temperature versus thermal energystorage temperature over 24 hours for a typical thermal energy storageand in the preferred embodiment;

FIG. 7 is a comparison of the preferred embodiment with a multi-stageflash distillation process;

FIG. 8 is a chart showing solar fraction and optimum solar panel areafor a typical absorption refrigeration system and in the preferredembodiment;

FIG. 9 is a chart showing desalination rates for different cooling loadsand solar panel areas for a typical absorption refrigeration system andin the preferred embodiment;

FIG. 10 is a chart showing the effect of withdrawal rate on desalinationefficiency and saline water temperature for the desalination system ofthe present invention;

FIG. 11 is a chart showing the effect of withdrawal rate onconcentration and desalination efficiency for the desalination system ofthe present invention;

FIG. 12A is a chart showing solar panel area versus desalination rate atdifferent withdrawals for the desalination system of the presentinvention; and

FIG. 12B is a chart showing cooling load versus desalination rate atdifferent withdrawals for the desalination system of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

This invention concerns the feasibility of a new desalination processutilizing low grade thermal energy. The process operates at near-vacuumpressures created by passive means. This allows for the process tooperate at low temperatures with higher efficiency. The process utilizeslow grade thermal energy rejected by an absorption refrigeration system(ARS). The condenser of the ARS rejects low grade thermal energy atapproximately 55° C. which is recovered and stored in a low temperaturethermal energy storage (TES) tank. A thermal energy storage tank servesbest to manage energy demands of the desalination process by maintainingthe source temperature at approximately 50-70° C. as the energy demandto the desalination system is dependent on the ambient temperature.

Results of this feasibility study show that the thermal energy rejectedby an ARS of cooling capacity of approximately 3.25 kW (0.975 tons ofrefrigeration) along with an additional energy input of approximately208 kJ/kg of desalinated water is adequate to produce desalinated waterat an average rate of approximately 4.5 kg/hr. Energy demand for thisprocess is competitive with well-established Multi Stage Flashdistillation processes. An integrated process model and performancecurves of the proposed approach are presented below. The effect ofprocess parameters, such as withdrawal rate, are also presented below.

The preferred embodiment of the desalination system of the presentinvention is shown schematically in FIG. 1. The invention comprisesdesalination unit 10, absorption refrigeration system 12, and thermalenergy storage 14. Desalination unit 10 comprises evaporation chamber16, condenser 18, one or more heat exchangers 20, 22, and columns 24, 26and 28 (e.g. between approximately 10-11 m tall, equivalent to the localbarometric head). These three columns are saline water column 24, brinewithdrawal column 26, and desalinated water column 28, each with its ownconstant-level holding tank, saline water tank 30, brine tank 32, anddesalinated water tank 34, respectively. The heat input to evaporationchamber 16 is provided by thermal energy storage 14, which is maintainedat a particular temperature or range (e.g. between approximately 50-70°C.) by absorption refrigeration system 12, preferably powered by solarenergy.

Holding tanks, saline water tank 30, brine tank 32, and desalinatedwater tank 34 are installed at ground level while, evaporation chamber16 is installed atop three columns 24, 26, 28 at a barometric height(e.g. between approximately 10-11 m, equivalent to the local barometrichead) above the free surface in holding tanks 30, 32, 34 to create anatural vacuum in the headspaces of three columns 24, 26, 28. Thisconfiguration enables the desalination process to proceed preferablywithout any mechanical pumping. There are also preferably no othermoving parts needed, except for the pump used to start the system. Thetemperature of the head space of the feed water column is maintainedslightly higher, approximately 45-65° C. than that of the desalinatedwater column, approximately 15-35° C. Since the head spaces are atnear-vacuum level pressures, a temperature differential (e.g. betweenapproximately 10-25° C.) is adequate to evaporate water from the feedwater side and condense in the desalinated water side. In this manner,saline water can be desalinated at between approximately 40-50° C.,which is in contrast to the 60-100° C. range in traditional solar stillsand other distillation processes. Brine is withdrawn continuously fromevaporation chamber 16 flowing through tube-in-tube heat exchanger 20preheating the saline water feed entering evaporation chamber 16.

Absorption refrigeration system 12 operates with a refrigerant (e.g.LiBr—H₂O) in a pressure range of preferably 1 to 16 kPa. Heat energyrequired by generator 38 of absorption refrigeration system 12 ispreferably supplied by solar collector 36 (e.g. flat panel) duringsunlight hours and by auxiliary electric or gas heater 40 duringnon-sunlight hours. Solar collector 36 maintains generator 38 at aspecified temperature or range (e.g. between approximately 80-110° C.).Heat rejected by condenser 42 of absorption refrigeration system 12maintains thermal energy storage 14 at a specified temperature or range(e.g. between approximately 50-70° C.) to serve as the low grade heatsource for the desalination process. Evaporator 44 of absorptionrefrigeration system 12 feeds the cooling load. Thus, the preferredembodiment of the present invention performs two functions of continuousdesalination and cooling with minimal amounts of external nonrenewableenergy input.

Mass and Energy Balances

The preferred embodiment was developed based on mass and energybalances. Thermodynamic analysis of the preferred embodiment wasperformed through computer simulations using Extend®, manufactured byImagineThat Inc., San Jose, Calif. and EES®, manufactured by F-ChartSoftware, Madison, Wis. simulation software.

Desalination System

An evaporator area of 5 m² and a height of 0.25 m were considered. Inall calculations, the reference temperature was assumed to be 25° C. Allheat exchangers were assumed to have 80% efficiency. The following massand heat balance equations apply to the different components:

Mass balance on water in the evaporation chamber:

$\begin{matrix}{{\frac{}{t}\left( {\rho \; V} \right)} = {{\rho_{i}{\overset{.}{V}}_{i}} - {\rho_{w}{\overset{.}{V}}_{w}} - {\rho_{e}{\overset{.}{V}}_{e}}}} & (1)\end{matrix}$

Mass balance on solute in the evaporation chamber:

$\begin{matrix}{{\frac{}{t}\left( {\rho \; C\; V} \right)_{s}} = {{\rho_{i}C_{i}{\overset{.}{V}}_{i}} - {\rho_{w}C_{w}{\overset{.}{V}}_{w}}}} & (2)\end{matrix}$

Heat balance for the evaporation chamber:

$\begin{matrix}{{\frac{\;}{T}\left( {\rho \; c_{p}V\; T} \right)_{s}} = {Q_{In} + {\left( {\rho \; c_{p}T} \right)_{i}{\overset{.}{V}}_{i}} - {\left( {\rho \; c_{p}T} \right)_{w}{\overset{.}{V}}_{w}} - Q_{E} - Q_{L}}} & (3)\end{matrix}$

Evaporation rate is expressed by (Bemporad, 1995):

$\begin{matrix}{q_{e} = {\frac{\alpha_{m}A}{\rho_{f}}\left\lbrack {{{f\left( C_{s} \right)}\frac{p\left( T_{s} \right)}{\left( {T_{s} + 273} \right)^{1/2}}} - \frac{p\left( T_{f} \right)}{\left( {T_{f} + 273} \right)^{1/2}}} \right\rbrack}} & (4)\end{matrix}$

where, p(T)=e^((63.042−7139.6/(T+273)−6.2558 ln(T+273)))*10² Pa

In the above equations, subscripts e, i, and w, represent evaporatoroutlet, inlet, and withdrawal conditions, respectively; f and srepresent freshwater and saline water respectively. The variables aredefined as follows:

-   -   V=volume of water in the evaporation chamber [m³]    -   {dot over (V)}=volumetric flow rate [m³/hr]    -   ρ=density [kg/m³]    -   c_(p)=specific heat capacity of saline water [kJ/kg−° C.]    -   C=solute concentration [%]    -   T_(s)=saline water temperature in the evaporator chamber [° C.]    -   T=temperature [° C.]    -   q_(e)=evaporation rate [m³/s]    -   A=area of the evaporation chamber [m²]    -   αm=an experimental coefficient [10⁻⁷−10⁻⁶ kg/m²-Pa-s-K^(0.5)]        (11)    -   f(C)=correlation factor for the presence of solute concentration        [%]    -   Q_(In)=energy input from thermal energy storage [kJ/hr]    -   Q_(L)=energy losses from evaporation chamber [kJ/hr]    -   Q_(E)=energy used for evaporation [kJ/hr]

Energy used for evaporation is given by:

Q _(E)=3,600ρ_(f) h _(L)(T _(S))q _(e)   (5)

where, h_(L)(T) is the latent heat of evaporation [kJ/kg] given by:

h _(L)(T)=[(3146−2.36(T+273⁰ K)]

The desalination efficiency, η_(d), is defined as:

$\begin{matrix}{\eta_{d} = \frac{m_{e}h_{L}}{\sum{Q_{In}\Delta \; t}}} & (6)\end{matrix}$

where,

-   -   m_(e)=mass of desalinated water produced over a period of time        [kg]    -   h_(L)=latent heat of evaporation at saline water temperature        [kJ/kg]    -   ΣQ_(In)Δt=energy provided by the thermal energy storage over a        period of time [kJ]

Expressions for density, enthalpy and pressure variations are presentedbelow.

Density variation with temperature and concentration is given as:

ρ(T,C)=ρ₀(1−β_(T) ΔT ₀+β_(C) ΔC ₀)

-   -   where, ΔT₀ and ΔC₀ are variations from reference property        values, of density.    -   β_(T)=5.10−4 C−1, thermal expansion coefficient    -   β_(C)=8.10−3%−1, solutal expansion coefficient

Effect of concentration on specific heat is given as:

c _(P)(C)=α₂ C+β ₂

-   -   where, α₂=−30.10 J kg−1 0C−1        -   β₂=4178.4 J kg−1 0C−1

Evaporation energy is given as:

Q _(e)=ρ_(f) h _(fg)(T _(s))q _(e) [kJ/hr]

Latent heat of evaporation is given as:

h _(fg)(T)=10³*[(3146−2.36(T+273⁰ K)]J/kg

The average heat transfer from the tips of the fins is given by(Rohsenow 1985):

Nu_(s)=cRa_(s) ^(b)

-   -   where Nu=Nusselt number, Ra=Rayleigh Number and b and c are        constants        -   b=0.29; c=0.44+0.12/ε; ε=Dco/Dfin    -   This equation is valid for 2<Ras<104 and 1.36<1/ε<3.73    -   Rayleigh number is given as (Incropera, 2002):

${Ra}_{s} = {\frac{g\; {\beta \left( {T_{co} - T_{a}} \right)}S^{3}}{\alpha\gamma}\frac{S}{D_{fin}}}$$\beta = \frac{1}{273 + T_{a}}$

-   -   where g=local acceleration due to gravity (m/sec²)        -   β=temperature coefficient, 1/° K.        -   α=thermal diffusivity, m²/s        -   Y=kinematic viscosity, m²/s        -   T_(a) and T_(co)=temperatures of the ambient and condenser            respectively, ° K.        -   S=distance between the successive fins, m

Condenser Calculations: Average heat transfer through the cylindersurfaces and fins is given by:

${Nu}_{s} = {\frac{{Ra}_{s}}{12\pi}\left\{ {2 - {\exp \left\lbrack {- \left( \frac{c\; 1}{{Ra}_{s}} \right)^{3/4}} \right\rbrack} - {\exp \left\lbrack {- {\beta \left( \frac{c\; 1}{{Ra}_{s}} \right)}^{3/4}} \right\rbrack}} \right\}}$where 1.67 < 1/ξ < ∞ β = 0.17ξ + exp (−4.8ξ)c 1 = [23.7 − 1.1(1 + 152ξ²)^(1/2) + β]^(4/3)

The rate of heat transferred from the condenser prime surface and finscan be calculated as:

Q _(c) =[h _(co,tip) NA _(f,tip)η_(f) +h _(co) NA _(f,sides)η_(f) +h_(co) A _(b)](T _(co) −T _(a))

-   -   where h_(co,tip), h_(co)=heat transfer through the tips and base        or sides        -   N=number of fins        -   A_(b), A_(f,sides), and A_(f,tip)=areas of base, fin sides            and tip respectively        -   η_(f)=efficiency of fins    -   For turbulent free convection for Ra>109,

Nu=c(Ra)^(0.333)

-   -   where c=0.10    -   For f_(in) efficiency (Donald Q. Kern, 1972):

φ=(ro−ri)3/2 (2h/kAp)1/2

ρ=(ri/ro)

-   -   where r_(i) and r_(o)=inner and outer radius of fins        respectively, m        -   h=heat transfer coefficient trough the fin, w/m²−° K.        -   k=heat transfer coefficient trough the surface, w/m−° K.        -   A_(p)=Area of the fins

η=(φ, ρ)

Absorption Refrigeration System

Absorption refrigeration system 12 is preferably driven by solar energyduring sunlight hours and by auxiliary electric or gas heater 40 duringnon-sunlight hours, although any type of power may be utilized. Theefficiency of solar collectors is expressed in terms of solar fraction,which is the contribution of solar energy to the total load in terms ofthe fractional reduction in the amount of extra energy that must besupplied. A storage tank volume of approximately 0.125 m³/m² wasconsidered and the optimum area of solar collectors required was foundfrom a solar fraction graph. The optimum number of collectors was thelowest number of collectors for which a 100% solar fraction was achievedat the hour maximum solar radiation. Additional energy for heating andpumping was required for condenser 42 of absorption refrigeration system12 to dissipate heat at approximately 55° C. The pumping requirementswere calculated using EES® software.

Heat balance across solar collection system:

$\begin{matrix}{\frac{\left( {m_{S}C_{ps}T_{S\; 1}} \right)}{t} = \begin{bmatrix}{{F_{R}A_{p}\left\{ {{({\tau\alpha})I_{S}} - {U_{L}\left( {T_{gs} - T_{a}} \right)}} \right\}} -} \\{{U_{S}{A_{S}\left( {T_{S\; 1} - T_{a}} \right)}} - {m_{R}{C_{pr}\left( {T_{S\; 1} - T_{gs}} \right)}}}\end{bmatrix}} & (7)\end{matrix}$

where, m_(s)=mass of water in storage tank [kg]

-   -   C_(ps)=specific heat of water in storage tank [kJ/kg−°C.]    -   T_(S1)=temperature of water in storage tank [° C.]    -   F_(R)=heat removal factor [dimensionless]    -   A_(p)=area of solar panels [m²]    -   T=transmitivity of glass [dimensionless]    -   α=absorptivity of water [dimensionless]    -   I_(S)=solar energy [kJ/hr−m²]    -   U_(L)=heat loss coefficient [kJ/hr−m²−° C.]    -   T_(gs)=temperature of the water from the generator [° C.]    -   T_(a)=ambient temperature [° C.]    -   U_(S)=heat losses from the surface of storage tank [kJ/hr−m²−°        C.]    -   A_(S)=surface area of storage tank [m²]    -   m_(R)=flow rate of recycling water [kg/hr]    -   C_(pr)=specific heat of recycling water [kJ/kg−° C.]

Thermal Energy Storage:

Sensible heat thermal energy storage 14 stores heat rejected by theabsorption refrigeration system-condenser 42. The optimal volume ofthermal energy storage 14 to maintain evaporation chamber 16 at thedesired temperature differential was determined by solving the heatbalance for thermal energy storage 14 by trial and error.

Heat balance for thermal energy storage 14:

$\begin{matrix}{{\frac{}{t}\left( {\rho \; C_{p}v\; T} \right)_{T\; E\; S}} = {Q_{R} - Q_{In} - Q_{L\; 1}}} & (8)\end{matrix}$

where, Q_(R)=heat rejected by condenser in the absorption refrigerationsystem [kJ/hr]

-   -   Q_(L1)=energy losses from the thermal energy storage surface        [kJ/hr]    -   C_(p)=specific heat of the water in the TES [kJ/kg−° C.]    -   v=volume of the thermal energy storage [m³]

Results:

The model equations were solved using the fixed parameters listed inFIG. 2 and for a particular site. Previous studies have shown that theeffect of water depth in evaporation chamber 16 did not have anysignificant effect on the evaporation rate. This is in contrast to thetraditional solar stills, where the water volume provided energy storagethat is required for continued evaporation during non-sunlight hours.Since the preferred embodiment does not depend on solar energy forcontinuous operation, the effect of water depth was not taken intoaccount.

First, results of an example case where the model equations were solvedfor the reference parameters listed in FIG. 2 are presented. In thisexample case, the withdrawal rate was fixed at approximately 2.5 kg/hr(≈50%). These results demonstrate the effectiveness of the preferredembodiment. Then, the total energy consumption of the preferredembodiment was analyzed and compared to that of a multi-stage flashdistillation process. Finally, the effect of withdrawal rate on thedesign and performance of the preferred embodiment is presented.

Heat Balance for Evaporation Chamber

The heat balance for evaporation chamber 16 is described by Equation 3.FIG. 3 shows the variations in heat provided by thermal energy storage14, the heat consumed for evaporation, and the heat lost over a 24-hrperiod for a summer day, when the ambient temperature ranged frombetween approximately 25 to 37° C. The desalination efficiency definedby Equation 6 is also indicated in FIG. 3 by the bold line. As expected,the heat lost by evaporation chamber 16 was higher during non-sunlighthours than that during sunlight hours due to lower ambient temperaturesduring non-sunlight hours. Under the example conditions, the energyavailable for desalination was about 12,500 kJ/hr (=3.45 kW) which wasthe waste heat rejected by condenser 42 in absorption refrigerationsystem 12. However, the net heat transfer was dependent on thetemperature gradient between the transfer medium and the heat source.The actual mass of water that could be evaporated in evaporation chamber16 and hence, the desalination efficiency, depended on the heat inputfrom thermal energy storage 14, the ambient temperature at which thecondensation took place, and the brine withdrawal rate. Since thedriving force for evaporation is the temperature differential, betweenevaporation chamber 16 and condenser 42, the heat input to evaporationchamber 16 during the day is lower than that input during the night.During the night, both the ambient temperature and the freshwatertemperature are low, favoring a higher desalination rate, thus resultingin higher heat input and vice versa.

The temperature variations in the saline water in evaporation chamber 16and the desalinated water with respect to ambient temperature over a24-hr period are shown in FIG. 4. The temperature of saline water variedfrom between approximately 43.5 to 46° C. and the ambient temperatureranged from between approximately 25 to 37° C. while the fresh watertemperatures ranged from between approximately 35 to 40° C. FIG. 4 showsthat thermal energy storage 14 was able to maintain the approximately10° C. temperature differential between the saline water side and thedesalinated water side. It is noted that the ambient temperature is animportant variable because condensation occurs at the ambienttemperature, which indirectly determines the desalination rate in thisprocess.

Operating Conditions of Absorption Refrigeration System

Absorption refrigeration system 12 is designed for two functions: formaintaining thermal energy storage 14 at the desired temperature and forproviding the cooling load. As such, absorption refrigeration system 12operates under slightly different conditions compared to the traditionalsystems used for cooling alone. Operating conditions for a typicalabsorption refrigeration system used in cooling and the conditions forabsorption refrigeration system 12 are compared in FIG. 5, for the samecooling load of approximately 3.25 kW. The notable difference is thepressure ranges between approximately 1 and 6.3 kPa versus about 1.4 to15.75 kPa respectively.

Volume of Thermal Energy Storage

Winter conditions were assumed to determine the volume of thermal energystorage 14 necessary to provide the heat energy to evaporation chamber16. Solving Equation 8 by trial and error so that the temperatures atthe beginning and the end of a 24-hr period would be withinapproximately ±0.01° C., the volume of thermal energy storage 14 wasfound to be approximately 10 m³. The heat demand by evaporation chamber16 on thermal energy storage 14 varied from between approximately 8,700and 14,200 kJ/hr over a 24-hour period shown in FIG. 2; yet, as shown inFIG. 6, thermal energy storage 14 volume of approximately 10 m³ wasfound to be adequate to maintain its temperature at approximately 50° C.throughout the same period to provide the energy needs of evaporationchamber 16.

Energy Requirements

An embodiment of the invention may require additional non-renewableenergy for the following: auxiliary heat energy for generator 38(=approximately 192 kJ/kg of desalinated water) plus mechanical energyto circulate heat transfer medium between thermal energy storage 14 andevaporation chamber 16 (=approximately 14 kJ/kg of desalinated water);to circulate the heat transfer medium between thermal energy storage 14and condenser 18 (=approximately 2 kJ/kg of desalinated water); and topump the refrigerant in absorption refrigeration system 12(=approximately 0.04 kJ/kg of desalinated water). Hence, the totaladditional energy required to maintain thermal energy storage 14 at thedesired conditions is approximately 208 kJ/kg of desalinated waterproduced. In comparison, multi-stage flash distillation process wouldrequire a heat energy of approximately 294 kJ/kg of desalinated waterplus a mechanical energy of approximately 44 kJ/kg of desalinated water,for a total of approximately 338 kJ/kg of desalinated water. Thus, thepresent invention requires about 60% of the energy required by themulti-stage flash distillation process. A comparison between the twoprocesses is summarized in FIG. 7.

Solar Collector for Absorption Refrigeration System

Solar collector 36, augmented by auxiliary electric heater 40, is sizedto provide for thermal energy storage 14 and the cooling load. Thetemperature of storage tank 46 of solar collector 36 is set to aparticular temperature or range (e.g. between approximately 110-115° C.)in order to maintain generator 38 temperature at a particulartemperature or range (e.g. between approximately 100-110° C.). Theenergy to be provided by auxiliary heater 40 is equal to the differencebetween the energy required by generator 38 and that can be collectedfrom solar isolation. FIG. 8 illustrates this difference and the solarfraction, over a 24-hr period. The optimal area of the collectors can befound from Equation 7. For the conditions described herein, solarcollector area of approximately 25 m² can satisfy a cooling load ofapproximately 3.25 kW, at an average desalination rate of approximately4.3 kg/hr. The relationships between desalination rate, solar panelarea, and cooling load are presented in FIG. 9.

Brine Withdrawal vs. System Performance

Brine withdrawal rate is the primary control variable in this system,which has positive as well as negative impacts on the performance of thesystem. At low withdrawal rates, salts build up in evaporation chamber16, and evaporation rates decrease as shown by Equation 4. High saltlevels also reduce the enthalpy of saline water that can further reduceevaporation. For example, when salinity increases by approximately 1%,evaporation is also reduced by about the same percentage. Even thoughbetter salt removal can be achieved with higher withdrawal rates, largeamounts of sensible heat are also removed from evaporation chamber 16,resulting in decline of evaporation chamber 16 temperature. Simulationresults presented in FIG. 10 show the decline in evaporation chamber 16temperature and in desalination efficiency with increasing withdrawalrate. For example, the desalination efficiency dropped from betweenapproximately 90.5% and 80% when the withdrawal rate increased frombetween approximately 2.5 kg/hr and 25 kg/hr. FIG. 11 shows the saltbuildup with time and the resulting decline in desalination rate.Similar observations have been reported previously.

Further simulations were conducted to evaluate the effect of withdrawalrate on desalination rate, cooling load, solar collector area, andauxiliary heat requirement. As shown in FIG. 12, cooling load and solarpanel area are not sensitive to withdrawal rate in the range of betweenapproximately 50-200%. For a given desalination rate, even though thecooling load at a withdrawal rate of approximately 200% is higher thanthat at approximately 100%, the auxiliary heat addition is also higher.In addition, the solar collector area is also higher. As discussedbefore, the desalination efficiency also decreases. Based on theseresults, a withdrawal rate of approximately 100% is an acceptable rateto minimize salt buildup and maintain system performance.

In summary, model simulations show that the preferred embodiment canachieve a desalination efficiency of at least approximately 85% andhigher (e.g. approximately 85-90%) at a brine withdrawal rate ofapproximately 70-100% with an energy consumption of less thanapproximately 250 kJ/kg (e.g. between approximately 150-300 kJ/kg) offreshwater from seawater. The energy requirements for the preferredembodiment are less than that are required for a multi-stage flashdistillation process. Based on the results from the example, anapproximate 100% withdrawal rate could prevent scale formation thatcould reduce the evaporation rate. A typical unit with a thermal energystorage volume, the volume calculated by solving a heat balance usingtrial and error (e.g. between approximately 10-20 m³), can produce freshwater at between approximately 4.5 kg/hr and provide a cooling load ofbetween approximately 3.25 kW with a solar panel area of betweenapproximately 25 m². The preferred embodiment minimizes non-renewableenergy usage and may be improved further by incorporating a double ortriple-effect configuration.

Although the invention has been described in detail with particularreference to these preferred embodiments, other embodiments can achievethe same results. Variations and modifications of the present inventionwill be obvious to those skilled in the art and it is intended to coverin the appended claims all such modifications and equivalents An exampleof a configuration that has been demonstrated by a prototype isdescribed in Example 1. The entire disclosures of all references,applications, patents, and publications cited above are herebyincorporated by reference.

EXAMPLE 1

A prototype unit was constructed and experiments were conducted usingdirect solar energy and photovoltaic energy as heat sources.Desalination was performed on a continuous basis over 24 hours a day forseveral months. This prototype comprised of columns that were 10 m,equivalent to the local barometric head. The temperature of the headspace of the feed water column was maintained at approximately 40-50°C., while the desalinated water column was maintained at approximately35-45° C. The pressure in the evaporation chamber remained atapproximately 0.085 atm. The specific energy required by this prototypewas approximately 3,370 kJ/kg of desalinated. This system was runentirely on solar energy with direct solar heat during sunlight hoursand with a 350-W DC heater powered by batteries that were charged by thephotovoltaic panels during the day time. This example system was able torecover potable quality water meeting United States EnvironmentalProtection Agency drinking water standards from the effluent of amunicipal wastewater treatment plant.

1. A desalination system comprising: a desalination unit; a low gradeheat source for powering said desalination unit; and a thermal energystorage system for storing the low grade heat source and maintaining aspecified temperature range.
 2. The system of claim 1 wherein saiddesalination system runs continuously.
 3. The system of claim 1 whereinsaid specified temperature range is between approximately 40-50° C. 4.The system of claim 1 wherein said low-grade heat source is at leastpartially supplied by an absorption refrigeration unit.
 5. The system ofclaim 4 wherein said absorption refrigeration unit maintains saidthermal energy storage system at said specified temperature range. 6.The system of claim 4 wherein said absorption refrigeration unitoperates at a pressure range of between approximately 1.4 to 15.75 kPa.7. The system of claim 1 wherein said desalination unit comprises anevaporation chamber, a condenser, a heat exchanger and one or morecolumns.
 8. The system of claim 7 comprising at least three columns, oneof said columns comprising a saline water column, another of saidcolumns comprising a brine withdrawal column and another of said columnscomprising a desalinated water column.
 9. The system of claim 8 whereinheat input to said evaporation chamber is provided by said thermalenergy storage system.
 10. The system of claim 1 not having a pumpexcept for an initial starting pump.
 11. The system of claim 10 nothaving any other moving parts.
 12. A method of desalinating comprisingthe steps of: operating a desalination unit; powering the desalinationunit using a low grade heat source; storing the low grade heat source ina thermal energy storage unit; and maintaining a specified temperaturerange of the low grade heat source.
 13. The method of claim 12comprising running the desalination unit continuously.
 14. The method ofclaim 12 comprising desalinating saline water at a temperature range ofapproximately 40-50° C.
 15. The method of claim 12 comprising supplyingthe low grade heat source at least partially by an absorptionrefrigeration unit.
 16. The method of claim 15 comprising providing acooling load from the absorption refrigeration unit.
 17. The method ofclaim 15 comprising maintaining the thermal energy storage unit at aspecified temperature range within the absorption refrigeration unit.18. The method of claim 15 comprising operating the absorptionrefrigeration unit at a pressure range of between approximately 1.4 to15.75 kPa.